Electromagnetic induction has been known for a long time and it has been used in many applications. In electromagnetic induction a time-varying magnetic flux induces an electromotive force to a closed conductor loop. Vice versa, a time-varying current creates a varying magnetic flux. In transformers, this phenomenon is utilized to transfer energy wirelessly from circuit to another via inductively coupled coils. A primary coil transforms an alternating current into a varying magnetic flux, which is arranged to flow through the secondary coil. The varying magnetic flux then induces an alternating voltage over the secondary coil. The proportion of the input and output voltage can be adjusted by the number of turns in the primary and secondary coils.
Wireless charging is another application where electromagnetic induction is used to transfer energy over air. A wireless charging system comprises a charger device with a primary coil, and a device to be charged with a secondary coil. The current in the charger device is transferred to the charged device through these electromagnetically coupled coils, and the induced current may be further processed and used to charge the battery of the charged device. Energy is transmitted through inductive coupling from the charger device to the charged device, which may use that energy to charge batteries or as direct power.
A trend and key selling point in today's portable electronic devices, e.g. in mobile phones and other smart devices, has been and continues to be the thinness of the device. However, demands of devices suitable for wireless charging are against this trend since those devices need induction coils and induction coils add thickness to device design.